1. Field of the Invention
The present invention relates to a device for transmitting driving power between two axes, which are relatively rotatable to each other.
2. Discussion of the Prior Art
While the basic function of a driving power transmission device used in a four-wheel drive system is to transmit driving power to rear wheels in response to the rotational speed difference between front wheels and rear wheels, ability of absorbing circulation torque, which occurs in only four-wheel drive vehicles, is also an important function. The circulation torque is a rotational torque, which occurs within a loop constituted by front wheels, a road surface, rear wheels, a propeller shaft, and the driving power transmission device, when rotational speed difference between the front wheels and the rear wheels appears due to running condition of the vehicle. If the rear wheels rotate faster than the front wheels because of the circulation torque, the circulation torque is transmitted from the rear wheels to the front wheels through the propeller shaft and the driving power transmission device, and causes vibration of the propeller shaft which is a cause of booming noise produced in the passenger room. Therefore, the ability of absorbing the circulation torque is one of important functions of such driving power transmission device. Such circulation torque can be absorbed if the rotational torque is not transmitted or is reduced when the rotational torque is transmitted from the rear wheels to the front wheels through the transmission device, i.e. the rear wheels rotate faster than the front wheels.
A driving power transmission device for the well known four-wheel drive vehicle, however, does not have any capability of decreasing the transmissive torque when the rear wheels rotate faster than the front wheels. Accordingly, the driving power transmission device in the prior four-wheel drive system cannot absorb the aforementioned circulation torque.
The circulation torque is also produced when an antilock brake system provided in the vehicle operates, and conflict between the antilock brake system and the driving power transmission device occurs. If the circulation torque is absorbed by the transmission device, the antilock brake system can operate correctly. Therefore, the ability of absorbing the circulation torque is also important in the four-wheel drive vehicle, which is provided with the antilock brake system.